Currently high voltage equipment, especially high voltage equipment in valve halls, such as valves of current converters may require regular maintenance and/or alteration. When the valve is energized no personnel is allowed to be in the valve hall due to risk of injury. For some purposes, such as maintenance, the valves need to be deenergized to allow personnel to enter the valve hall. In case personnel has to enter the valve hall, the valves need to be connected to ground to avoid electric discharges from the valves, such as surge current strokes or corona discharges originating from residual capacitor charges or static charge in the insulating material, which may harm the personnel present in the valve hall.
It is known to provide grounding of the valves when the valve hall is open and accessible for maintenance personnel and people in general, thus when the valves are deenergized. The grounding is used to ensure that residual charges in the system or arrangement, for example from the capacitors, are discharged. For such grounding purposes an operator has to enter the valve hall and manually install a grounding contact. Such a grounding contact must first be moved to the right place, then connected to a plug or socket in the valve hall floor and then extended to establish electric contact with the valve. These steps have to be taken by an operator whom is comparably close to the actual grounding contact or whom is at least in the valve hall, since the grounding contact has to be manually moved in the right position, connected to the plug and extended to electrically connect to the valve. The operator is thus exposed to the risk of contacting the two terminals of a capacitor, when the capacitor is still charged and this may be very harmful for the operator. In addition remaining charges in the capacitors may result in spontaneous discharges during these steps, which may cause deadly injuries. In addition, such a grounding operation involves a plurality of manual steps, which raises potential risk of failures or mistakes. Further, the above described manual steps are time consuming and thus costly.
In particular the application of a manual grounding process and thus of a manual grounding system in modular multilevel voltage source converter system comprising a plurality of cells is difficult, cumbersome and involves time consuming work.
Prior art grounding systems do not provide a possibility to ground a valve automatically or electrically control or check the grounding of the valves when the grounding system is in place, prior to the personnel entering the valve hall. In known systems there is thus always a remaining risk that a whole valve or a valve cell is not correctly connected to ground and that electric charges remain somewhere in the system.